1. Field of Invention
The present invention relates to the field of analog integrated circuits and mixed signals integrated circuits, particularly, to a CMOS bandgap reference source circuit with low flicker noises.
2. Brief Description of Related Arts
Bandgap reference source circuit is an important part of the analog integrated circuit and is widely applied to various analog and mixed signal integrated circuits, such as switching power supply (DC-DC), linear regulator, digital-analog converting circuits (ADC&DAC) and so on, which all need a reference voltage without changing along with the power supply and temperature. Among various reference circuits, the bandgap reference source circuit has better temperature characteristic and power supply rejection ratio (PSRR), is not subject to manufacturing techniques, and accordingly becomes the first option for designing the reference circuit.
For some system circuits requiring a reference voltage with high accuracy, a CMOS bandgap reference source circuit with low noises is necessary. Two main noise sources with the CMOS bandgap reference result from the flicker noises of field effect transistors (FET) and thermo-noise of all components in circuits. Generally speaking, the flicker noises of FET is inversely proportional to the frequency. When the frequency is about dozens of KHz, the flicker noise becomes the main noise source of the CMOS bandgap reference circuit. Consequently, bigger flicker noises have limited the application of the CMOS bandgap reference circuit. For example, the high performance audio DAC circuit working at the range of 20 Hz to 20 KHz really needs a low noise reference circuit to ensure the performance of the converting circuit.
FIG. 1 shows a conventional CMOS bandgap reference source circuit with the following operational principle. The circuit adopts the feedback control of the operational amplifier to make the voltage of node N1 and N2 the same value, thereby the current flowing through resistor R1 is equal to ΔVbe/R1, where ΔVbe is the difference of Vbe0 minus Vbe1. FET MP1, MP2 and MP3 compose a current mirror. Since the gate-source voltages of the three FET are the same and three of them all work at saturation regions, their drain-source currents are approximately the same. Therefore, the output of the reference circuit is
      V    ref    =            V              be        ⁢                                  ⁢        2              +                            R          2                          R          1                    ⁢      Δ      ⁢                          ⁢                        V          be                .            In the equation, Vbe2 is negative temperature coefficient and ΔVbe is positive coefficient, so that the output voltage with a zero temperature coefficient can be attained by setting up the ratio of R2 and R1.
For the CMOS bandgap reference source circuit shown in FIG. 1, when operating at the audio range, the two input transistors MP8 and MP9 generate flicker noises that become the main noise source. There are a couple of conventional ways to reduce the flicker noises.
(1) increase components area to reduce flicker noises
According to the definition for flicker noises, the noise density of flicker noises is given by the following formula:
            V      n      2        =                  K                              C            ox                    *          W          *          L                    *              1        f              ,wherein K is a constant with the order of magnitude of 10−25 V2F, not subject to manufacturing techniques; Cox is capacitance of gate oxide per unit area; f is operating frequency; W is the width of FET; L is the length of FET. It can be seen from the formula that flicker noise is inversely proportional to frequency. The smaller the frequency, the bigger the noises. In addition, flicker noise is proportional to the area (W*L) of the FET, therefore, the easiest way to reduce flicker noise is just to expand the area of components. However, this way has led to bigger chips areas, especially when a system requires a reference voltage with lower flicker noises. For instance, suppose there is a ADC with SNR of 100 dB and the voltage of output signals is 1V. To achieve 100 dB SNR, the total maximum noise is 10 uV; therefore, the noise resulting from reference source must be less than 10 uV. To generate that little noise, each of components in the circuit has to expand their areas close to one thousand of square micron.
(2) utilize CHOP structure to average flicker noises
CHOP structure is used to make the flicker noise be equivalent to an offset voltage of a reference circuit, to switch periodically two input terminals and two output terminals of the operational amplifier, to average the power spectrum of the flicker noises in a certain frequency range, and then to achieve a reference voltage output with lower noises through a lowpass filter. However, because the lowpass filter is implemented through resistors and capacitors inside of the chips, it would also cause bigger chips area.
(3) by way of special manufacturing techniques
U.S. Pat. No. 6,514,825, U.S. Pat. No. 6,160,274, U.S. Pat. No. 6,653,679 all use special manufacturing techniques to manufacture FET with low flicker noise. Nevertheless, these methods are relatively complicated, don't fit the mainstream standard of CMOS technology and have increased manufacturing cost as well.